A lithium secondary battery is a secondary battery which has high charge-discharge capacity and can exhibit high power. Currently, a lithium secondary battery is mainly being used as a power source for portable electronic appliances, and is expected to be used as a power source for electric automobiles predicted to be used more in the future. However, when a lithium secondary battery is used as a power source for portable electronic appliances, particularly, a power source for automobiles, it is required to reduce costs and space. Further, a lithium secondary battery currently and mainly being used as the power source for portable electronic appliances is also being required to become short, small, light and thin.
Among currently-used lithium secondary batteries, lithium secondary batteries, which are manufactured using rare resources, called rare metals such as cobalt, nickel and the like, as cathode materials, are being chiefly used. Therefore, battery materials advantageous in terms of resources are desired.
Sulfur is a resourceful and inexpensive material. Moreover, when sulfur is used as a cathode active material for a lithium secondary battery, it is theoretically expected that the cathode active material is a material having a maximum capacity among well-known cathode materials and that the cathode active material has an electric capacitance about six times larger than that of a lithium cobalt oxide cathode material which is the most frequently used among currently commercially-available cathode materials. Therefore, it is required to put sulfur to practical use as a cathode material.
However, a compound of sulfur with lithium is soluble in a nonaqueous solvent, such as ethylene carbonate, dimethyl carbonate or the like, which is used as a nonaqueous electrolyte for a lithium secondary battery. Therefore, when this compound is used as a cathode material, there is a problem in that this compound is gradually deteriorated by the elution of the composition to an electrolyte, thus decreasing the capacity of a battery. For this reason, methods of preventing the elution of the compound to an electrolyte using a polymer electrolyte or a solid electrolyte have been reported. However, these methods are also problematic in that the electric resistance of a battery becomes high, so that it is difficult to operate the battery at room temperature or a low temperature, with the result that the battery must be operated at high temperature, and the power of the battery becomes low.
Therefore, if the elution of sulfur to a nonaqueous solvent can be prevented and a sulfur-containing material can be practically used as a cathode material of a lithium secondary battery, it is possible to increase the capacity of a lithium secondary battery, decrease the weight thereof and reduce the space thereof. Further, if an electrolyte composed of a nonaqueous solvent, not a polymer electrolyte or a solid electrolyte, is used, it is possible to operate the battery even at room temperature or a low temperature.
As an attempt to prevent the elution of sulfur to a nonaqueous solvent, a sulfur-containing polymer having a —CS—CS— bond or a —S—S— bond has been proposed (refer to non-patent document 1 below). However, when this sulfur-containing polymer is used as a cathode material, lithium (Li) bonds with sulfur (S), so that the polymer is cut, thereby losing reaction reversibility. Therefore, there is a problem in that the cycle life characteristics of a battery deteriorate.
Further, a polymer lithium battery including carbon polysulfide essentially consisting of carbon and sulfur is disclosed in patent document 1 below. Such a polymer lithium battery including the carbon polysulfide is considered to have good stability and excellent charge-discharge cycle life characteristics. However, in the case of Example 9 in which aluminum foil is used as a collector, it cannot be expected that the cycle life characteristics of the polymer lithium battery were sufficiently improved, considering that the discharge capacity of the polymer lithium battery was 610 mAh/g per active material at the 10th charge-discharge cycle, whereas the discharge capacity thereof was decreased to 146 mAh/g at the 50th charge-discharge cycle. The reason for this may be that the carbon polysulfide has a structure in which sulfur is added to a straight-chain unsaturated polymer, so that the —CS—CS— bond and/or —S—S— bond of the carbon polysulfide is easily cut, with the result that the carbon polysulfide is converted into a low-molecular-weight polymer, thereby causing the low-molecular-weight polymer to be dissolved in an electrolyte, during the charge-discharge cycles.
Further, there is a problem in that it requires a multi-step process and a lot of time to synthesize the carbon polysulfide because the synthesis method of the carbon polysulfide is very complicated. Moreover, the carbon polysulfide does not have sufficient conductivity. Therefore, when the carbon polysulfide is used as a cathode active material, there are problems in that it is required to add a large amount of an auxiliary conductivity agent and in that the capacity of the polymer lithium battery per electrode weight becomes low.